Abstract
Purpose :
The single-file flow patterns of red cells passing through human retinal capillary networks are currently not well understood. We sought to determine, in normal healthy eyes, the degree to which adjacent foveal capillary segments support similar flow patterns, and how flow through any one segment can change over the course of hundreds to thousands of cardiac cycles.
Methods :
The foveal microvascular networks of 2 healthy individuals (22-23 years) were imaged at 300 fps for 3 seconds using a 750nm flood-illuminated adaptive optics ophthalmoscope. Despite the instrument’s small field of view (~1.25°), fixational eye movements were sufficiently small so that isolated segments of capillaries remained in view over several cardiac cycles. For 6 such segments, individual red cells were manually tracked to determine how flow velocity evolved over time. These ‘gold standard’ tracings served to validate a semi-automated kymograph approach to create velocity profiles for 24 segments, 7 of which were imaged up to 5 times separated by 2 to 38 minutes. When velocity exceeded 3 mm/sec (~15% of cases), temporal under-sampling lead to aliased flow so these segments were not analysed.
Results :
Pulsatile flow was found to be a universal feature for all capillary segments, with overall average velocity 0.80 mm/sec (SD=0.27). Average peak velocity (Vmax) was 1.51 mm/sec but was variable, ranging within any one field from 0.72 to 2.47 mm/sec. Similarly, trough velocity (Vmin) averaged 0.71 mm/sec overall but varied within a single field from 0.29 to 1.28 mm/sec. Velocity waveforms were universally asymmetrical with trough-to-peak-time/period ratios (“abruptness”) averaging 0.21 (range 0.11-0.39). Pulsatility contrast (Vmax-Vmin / Vmax+Vmin) averaged 0.38 overall and ranged from 0.20 to 0.68; this was more consistent within any one field with greatest range 0.20-0.42. Adjacent velocity waveforms maintained synchronicity with dispersions amongst the times of peaks and troughs generally less than 100 msec. Individual segments imaged multiple times maintained relative consistency in all waveform parameters, with greatest ranges in Vmax, Vmin, abruptness, and pulsatility contrast recorded as 1.75-2.14 mm/sec, 0.31-0.48 mm/sec, 0.19-0.33 and 0.38-0.52 respectively.
Conclusions :
This analysis indicates a higher degree of spatial relative to temporal heterogeneity in the characteristics of red cell flow through foveal capillary networks.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.